Dear Attorneys, I am on L1A visa with my wife and kids. My employer has applied for I-94 extension last month.

Yes your application has been approved. You should receive confirmation on your I-94 approval soon.

What happens if I overpayed my filing fee for citizenship

Immigration will return the fee and application because the fee is not correct.

Will USCIS and/or the immigration judge believe me that I am a victim of immigration marriage fraud and blackmail for green card

You can report this to USCIS fraud unit. Whether or not they believe you depends on what evidence you have to support your allegation.

Can I work with a current EAD but an expired L2 visa?

Check your I-94 to see when your permission to be in the US expires. The EAD usually expires at the end of the period granted on the I-94.

What visa should my husband use to enter the country?

A same sex civil union does not equal a marriage. You must have been legally married in a state that allowed the same sex marriage before you got...

Am I automatically granted 6-year H1B at each degree level in USA?

No you are not automatically granted a 6 year H-1b at each degree level in the US.

Yes you can get your driver permit. Your parents cannot help you with the driving lessons though if they do not have a US driver license.

Possibility and procedure to activate H1B petition after returning to home country before utilizing all the 6 years

There are time limits for using the cap exemption. You have been out of the US too long to recapture unused time on the old visa.

Applicability of correct Canadian Permanent Residency scheme towards immigration there in Canada.

This is really a question for a Canadian Immigration Attorney. This forum focuses on US law.

I got my F1 stamped in US Embassy in India but I cannot join this semester. Can I use the same F1 or will I have to reapply?

You will have to reapply. That visa is only good for the period and semester that you were supposed to start school.